The invention generally relates to a dual mode transmitter, such as a transmitter for a serial bus, for example.
There are at least two signaling techniques that may be used to communicate bits of data over a serial bus. Referring to FIG. 1, one technique, called single-ended voltage mode signaling, uses changes in a voltage of a serial bus line to indicate bits of data. As an example of this type of signaling, a voltage signal 10 on the serial bus line may swing between a high voltage level (called VMAX) to indicate a logic one bit and a lower voltage level (called VMIN) to indicate a logic zero bit. Due to the simplicity of the circuitry that is associated with voltage mode signaling, this type of signaling typically is used for bit rates less than about 50 megabits/second (Mb/s).
For higher bit rates (bits rates greater than about 50 Mb/s), a signaling technique called differential current mode signaling may be used. Typically, differential current mode signaling is associated with better signal integrity, lower switching noise and lower power dissipation. Referring to FIG. 2, with differential current mode signaling, two current signals 14 and 16 (present on two different serial bus lines) vary between a maximum current level (called IMAX) and a minimum current level (called IMIN) to indicate data bits. In this manner, the current signals 14 and 16 indicate a logic one bit when the current signal 16 has an IMAX current level and the current signal 14 has an IMIN current level. The current signals 14 and 16 indicate a logic zero bit when the current signal 14 has an IMAX current level and the current signal 16 has an IMIN current level.
A serial bus device typically includes a transmitter and a receiver to communicate with each serial bus data line using, for example, the single-ended voltage mode or the differential current mode signaling technique. As examples, the Universal Serial Bus (USB) standard uses the single-ended voltage mode signaling technique, and the USB 2.0 standard uses the differential current mode signaling technique. The USB standard is described in the Universal Serial Bus Specification, Revision 1.0, published in January 1996. The USB 2.0 standard is described on the Internet at www.usb.org/developers/index.html. To accommodate the two different serial bus standards, the serial bus device may include two transmitters: one transmitter for single-ended voltage mode signaling and another transmitter for differential current mode signaling. Unfortunately, two transmitters (instead of one transmitter) for each serial bus data line may significantly increase the fabrication area of the serial bus device. Furthermore, the performance of high speed transmissions may be affected due to the extra capacitances that are present at an output port of the serial bus device due to the presence of the two transmitters.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.
In an embodiment of the invention, a method includes selecting either a current signaling mode or a voltage signaling mode to communicate with a serial bus. When the current signaling mode is selected, an output stage is placed in the current signaling mode, and when the voltage signaling mode is selected, the output stage is placed in the voltage signaling mode.
Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims.